It is known to react methane and hydrogen in a plasma to synthesize C.sub.2 hydrocarbons (ethane and ethylene) directly. The plasma may be obtained in a microwave oven and may be initiated by metals such as Ni, Fe or Co, finely divided, and by mixtures of these metals, also highly divided.
The disadvantage of this process is that, on the one hand, much energy is needed to maintain the plasma and, on the other hand, the efficiency of obtaining C.sub.2 molecules is low, despite the technical improvements which it has been possible to propose (as in U.S. Pat. No. 4,574,038).
Another process consists in partially and catalytically oxidizing methane to synthesize ethane according to the reaction (1): EQU 2CH.sub.4 +1/2O.sub.2 .fwdarw.C.sub.2 H.sub.6 +H.sub.2 O
and then ethylene according to the reaction (2): EQU C.sub.2 H.sub.6 +1/2O.sub.2 .fwdarw.C.sub.2 H.sub.4 +H.sub.2 O.
The disadvantage of this method lies in the fact that the oxidation is difficult to restrict to the above-mentioned products which are formed and that it also gives rise to carbon monoxide or carbon dioxide according to the reaction (3): EQU CH.sub.4 +(1+x/2)O.sub.2 .fwdarw.CO.sub.x +2 H.sub.2 O
with x=1 or x=2.
Ethane and ethylene can also be oxidized to CO.sub.x as soon as they are produced during reactions (1) and (2). Consequently, this oxidation restricts the efficiency of these reactions, which take place on catalysts such as the oxides: La.sub.2 O.sub.3, Sm.sub.2 O.sub.3, NaPbO, MgO and BaO at a temperature of at least 600.degree. C. and in most cases in the gaseous phase.
Until now, and irrespective of the methane oxidation process employed, it has not been possible to obtain sufficient yields with high selectivities for the reactions used. At present the yield is at a ceiling of 16% and the selectivity never exceeds 80%. The latter is high only in the case of low degrees of progress of the reactions and, whatever the catalysts employed, it falls when an attempt is made to increase the degree of progress of the reaction by choosing optimum experimental conditions of temperature and pressure.
The yield is defined as being the ratio of the number of molecules of methane (CH.sub.4) necessary to form the product (that is to say ethane and ethylene or C.sub.2 molecules) to the number of molecules of CH.sub.4 entering the plant.
Selectivity (S) means the total selectivity for the product, defined as being the ratio of the number of molecules of CH.sub.4 converted into C.sub.2 molecules to the total number of converted CH.sub.4 molecules.
The degree of progress of the reaction, also called degree of conversion of methane, measures the total usage of methane. It is defined as being the ratio of the number of converted CH.sub.4 molecules to the number of CH.sub.4 molecules entering the plant.
These measurements can be made by chromatography.
It is recalled that the controlled catalytic oxidation of methane is a complex reaction which takes place in a number of stages:
--adsorption of the methane molecules on the catalytic sites of the solid catalyst, PA1 --conversion of the methane molecules into CH.sub.3 radicals in contact with the O.sup.- ions of the solid, to give OH groups, according to the reaction: EQU CH.sub.3 (gas)+O.sup.- (surface).fwdarw.CH.sub.3 +OH (surface), PA1 --combination of the CH.sub.3 radicals to give ethane according to the reaction: EQU 2.CH.sub.3 .fwdarw.C.sub.2 H.sub.6 PA1 --controlled oxidation of ethane to give ethylene according to the reaction: EQU C.sub.2 H.sub.6 +1/2O.sub.2 .fwdarw.C.sub.2 H.sub.4 +H.sub.2 O PA1 E.sub.i =activation energy PA1 R=constant for perfect gases PA1 T=absolute temperature.
with elimination of the OH groups at the surface of the catalyst in the form of water, which creates gaps bridged by the absorption of oxygen,
Simultaneously, as a result of the presence of oxygen, methane, ethane and ethylene can be partially oxidized, according to the following reaction scheme: ##STR1##
with x=1 or x=2,
each of the reactions shown diagrammatically above taking place individually at its own reaction rate K.sub.i, calculated according to the Arrhenius law: EQU K.sub.i =k.sub.i .multidot.exp- (E.sub.i /RT)
with k.sub.i =rate coefficient
The objective of the invention is to remedy the disadvantages of the prior art and especially to propose new catalysts making it possible to promote the reaction mechanisms resulting in the formation of ethane and ethylene, while reducing those resulting in the formation of CO and CO.sub.2.